GEOPHYSICS AS AN INTEGRAL PART OF THE AQUIFER STORAGE AND RECOVERY PROCESS

Thomas L. Dobecki (1) and Jennifer L. Hare (2)

1 SDII Global Corporation, 4509 George Road, Tampa, FL 33634
2 Zonge Engineering & Research, 3322 E. Ft. Lowell Rd. Tucson, AZ 85716

Aquifer storage and recovery (ASR) has very many basic similarities, unknowns, and needs as compared with a variety of in situ processes that have been
investigated in the past. These include, among others, pumped storage, secondary and tertiary petroleum recovery (hydraulic fracturing, CO2 flooding, brine
flooding), and in situ combustion processes (coal gasification, oil shale retorting). A central concern to each of these as well as ASR is an accurate depiction of
how/where the process develops (process monitoring). Geophysical surveying techniques, principally methods relying on the electrical resistivity contrast
between native and stored waters, are viewed as the most probable means for mapping the shape and progress/growth of injected water during ASR activities.
Model studies are presented using typical ASR depths, formation (ambient) water and injected water resistivities, which indicate that electrical resistivity-based
surveys (e.g., controlled source audio magnetotelluric profiling [CSAMT] and transient electromagnetic sounding [TEM]) have the potential to track growth
and shape of the ASR injected waters.



 THE EFFECTS OF HETEROGENEITY OF THE UPPER FLORIDAN AQUIFER ON ASR SYSTEMS

Bill Hutchings (1), Vacher, H.Len (2), and Budd, David (3)

1 HSA Engineers & Scientists, 4019 East Fowler Avenue, Tampa, FL 33617
2 Geology Department, University of South Florida, 4202 E. Fowler Ave., Tampa, FL 33620
3 Department of Geological Sciences, University of Colorado, Campus Box 250, Boulder, CO 80309-0250

 The matrix permeabilities of approximately 1200 meters (m) of the Upper Floridan aquifer (UFA) in the southern SWFWMD area were measured
from cores in eight wells with a minipermeameter at one-foot intervals and classified by depositional texture. A wide range of lithologies ranging from
permeable grainstones to low-permeability high-mud packstones are present. The bulk of the intrinsic transmissivity is contributed from the grainstones and
dolostones, although they represent a minor percentage of the thickness of the aquifer.
 We selected a 200-foot interval from the Suwannee Limestone as a representative section to study the effects of bed-scale, layered heterogeneity on
a theoretical Aquifer Storage and Recovery (ASR) well. The wide range of permeabilities at the core scale was modeled with a highly discretized 2,, I I .
three-dimensional flow (MODFLOW) and solute-transport (MT3D) model to ascertain the distribution, storage, and recovery of injected water. The models
reveal that units of high permeability facilitate the depth of penetration of injected water into the aquifer. The domain of injected water is not at all like a
bubble, but instead much like a bottle brush. Taking into account the effects of buoyancy, which we did not do in this study here, one can easily picture the
inverted Christmas tree proposed by Missimer and associates for the geometry of the invaded domain.


 THE RELATIONSHIP BETWEEN PYRITE STABILITY AND ARSENIC MOBILITY DURING AQUIFER
 STORAGE AND RECOVERY IN SOUTHWEST CENTRAL FLORIDA

Gregg Jones (1) and Thomas Pichler (2)

1 Southwest Florida Water Management District, 2379 Broad St. Brooksville Fl, 34609
2- University of South Florida Geology Department 4202 East Fowler Ave SCA 532 Tampa, Fl 36620

Elevated levels of arsenic are common in water recovered from aquifer storage and recovery (ASR) systems that store oxygenated surface water in southwest
central Florida. Mineralogical investigations of the Suwannee Limestone, the preferred storage zone for ASR systems, have shown that the highest
concentrations of arsenic are associated with framboidal pyrite in zones of high moldic porosity.

This investigation employed geochemical modeling to examine the stability of pyrite in limestone during simulated injections of oxygenated surface water.
Injections were simulated for 20 wells with intervals in the Suwannee Limestone with known chemical composition. The goal was to determine if aquifer
redox conditions could be altered to the degree of pyrite instability. Increasing amounts of injection water were added to the formation water in a series of steps
and the resulting reaction paths were plotted on pyrite stability diagrams. The pre-mixing formation water in the wells plotted within the pyrite stability field
indicating that redox conditions were sufficiently reducing to allow for pyrite stability. Since arsenic is immobilized in pyrite, its concentration in the
formation water should be low. This was corroborated by actual analysis of arsenic in water samples; none of the 20 wells sampled had concentrations above
0.1 gg/1. During simulation, however, as the ratio of injection water to formation water increased, redox conditions became less reducing and pyrite became
unstable. As a result, arsenic would be released from the aquifer matrix. The simulation also showed that the ratio of injection water to formation water
necessary to cause pyrite instability was highly variable and seemed to be controlled by the chemical composition of the formation water.